The invention relates to AC motor drive control, in general, and in particular to static slip-recovery drives.
Static slip-recovery drives are known and have been found to be advantageous for many specific applications. Slip-recovery is a technique generally used with an induction motor of the wound-rotor type. With this type of motor, definite advantages obtain as contracted with the conventional squirrel cage induction motor, for instance. Thus, when a variable voltage, variable frequency AC power supply is not available, or such complexity is not desired, a variable speed drive can be achieved by controlling the rotor current. This approach which involves a controlled return to the network of the energy not used by the load, has been known with an additional machine as the Scherbius system, or as the Kramer system. With the advent of SCR power switches, control of the rotor current and slip-recovery have been practiced statically. The method consists in rectifying the AC current induced as secondary in the rotor of the motor and in creating in the DC link a counter-electromagnetic-force voltage, opposed to the rectified DC voltage, through coupling of an inverter between the AC power supply and the DC link from the rotor. SCR control provides speed and torque control of a wound-rotor motor having a stator powered from a constant voltage fixed frequency source. See for instance "Principles of Inverter Circuits" by B. D. Bedford and R. G. Hoft, page 404, FIG. 1154, John Wiley 1964. See also: Proc. IEE, Vol. 110, No. 8, August 1963, "Switching Drive of Induction Motors" by M. S. Erlicki and Y. Wallach, pp. 1441-1450; IEEE transactions PAS-85, No. 1, January 1966, "Inverter Motor Speed Control With Static Inverters in the Rotor", pp. 76-84; and IEEE transactions IGA-5, No. 1, Jan./Feb., 1969, "Slip Power Recovery in an Induction Motor by the Use of a Thyristor Inverter" by William Shepherd and Jack Stanway, pp. 74-82.
The static drive of the type described in this prior art is similar in many ways to a DC motor drive and can be used to control the speed of a wound-rotor AC induction motor. The AC current of the rotor is rectified and passed through the thyristor inverter. Ultimately, the rotor power is fed back into the AC line of the network. Generally, a transformer is used to couple the AC side of the inverter to the network. The motor is controlled by controlling the gating angle of the thyristors, therefore, the back EMF introduced in the DC link by the inverter, and ultimately the current in the rotor. In other words, by controlling the DC link voltage, the DC link current is controlled and, thus the AC current in the wound-rotor. Accordingly, the torque is being controlled. The regulator of such slip-recovery system includes two nested loops: a current inner loop and a speed outer loop.
Such motor drives are particularly useful for pump and fan drives, since these generally operate at a predetermined constant speed. However, motor operation is not continuous and there is a frequency cycling from zero speed to full speed and back. Such abrupt changes in and out of normal operation are extremely unfavorable in several respects.
With the slip-recovery drive, the power factor is worst at full speed. Moreover, the transformer experiences overheating with the current reflected back into the AC line, unless it is designed for a higher rating, which, however, entails a bulky and costly structure. While power factor correction can be successfully achieved for a given speed, recurrent changes in speed makes such correction less effective at speeds other than the designed speed.
Accordingly, an object of the present invention is to control a wound-rotor induction motor through a wide range of speed while maintaining a good power factor at substantially all speeds.
Another object of the present invention is to minimize losses in the transformer, associated with the inverter and the power supply, which is inserted in the slip-recovery loop of a wound-rotor induction motor drive system.
Still another object of the present invention is to minimize reactive power in a wide range of speed with a slip-recovery induction motor drive system.
A further object of the present invention is to reduce the filtering requirements in the DC link of a slip-recovery induction motor drive system.